There are around 100 billion stars in the Milky Way. Assume that about 1/4 of them are "sunlike" enough to allow life to evolve.
What are the odds that a civilization will be on any particular one of these stars now? A sunlike star will last about 10 billion years. Let's assume that the "average" civilization lasts about 10,000 years after spaceflight. (Remember, we can close to destroying ourselves only a decade or so after this). That means that any particular life-supporting start has a 10,000/10,000,000,000 chance of having a space-faring civilization now. That works out to a chance of about 0.0001%.
Given this, for each spacefaring civilization, there are 999,999 life-supporting stars with no civilization.
With that in mind, there seems little reason why a spacefaring civilization would want to bother invading another, with all those empty worlds out there.
Note that this makes certain assumptions. For example, it assumes that all sun-like stars will have earth-like planets. That's probably not the case. That means even fewer spacefaring civilizations. It also assumes that spacefaring civilizations exist for millenia. That may well not be true. Given the logorithmic nature of human progress, it is hard to imagine how this would be the case without technological stagnation.
Given that nature of progress, it is also hard to imagine how the human race could ever hope to stand up to a civilization that had been in space for millenia (which, if the average civilization lifetime is 10,000 years, would usually be the case). It would be like a group of 15th century conquistadors hoping to hold off the US Marines. Given that a technological advantage of only ten to twenty years in the Gulf War resulted in the utter annihalation of one side, well...
So, taking this seriously, I have to say that such attempts at defense are pointless. The chance of needing it are far too slight and the chance of it actually working are even slighter.
This is k5. We're all tools - duxup